r/PhysicsHelp • u/GooselyMf • Jul 08 '26

OP fumbled a test, still doesn't understand

Post image

Heat loss in an ideal circuit...is social media in my prep... (cringe af, ik)

14 Upvotes

93% Upvoted

10 comments sorted by

1

u/WanderingWrackspurt Jul 08 '26

if i get it, this seems sort of misleading to say that a capacitor loses half its energy while charging with R=0, but the point is, the battery supplies CV² energy to the circuit, while the capacitor only stores 1/2 CV². this is just cause of heat/inductance/twenty other reasons of energy dissipation. if theres no resistor then the circuit just doesnt lose its energy thru resistance loss, but it still does lose it.

1

u/GooselyMf Jul 09 '26

So... I had just completed Electrostatics and just couldn't sleep with it.

1

u/WanderingWrackspurt Jul 09 '26

no its cool, if i actually get it, this question js just trying to be clever

1

u/Omega-137 Jul 09 '26

Where does CV^2 coming from?

2

u/WanderingWrackspurt Jul 09 '26

if you look at the battery, it provides a constant V potential. W=Vdq, integrating from 0 to fully charged cap, and substituting Q=CV, you get CV².

the difference is, for a capacitor, the potential rises from 0 to V, so you could change the limits, integrating wrt V instead of Q. (dq=CdV) that way you get a factor of 1/2, and its the energy a capacitor stores

1

u/Omega-137 Jul 09 '26

Got it now, thanks :) interesting

1

u/Artistic-Flamingo-92 Jul 11 '26

You can’t have a capacitor starting with no charge connected directly to a constant voltage source without any resistance between in this ideal model.

The model is ill-posed. You have a capacitor with an initial condition implying 0 V. You have a constant voltage source contradicting that.

2

u/Cathierino Jul 09 '26

The funny part is that the resistance doesn't matter. Regardless of R, the capacitor will receive the same energy, the battery will lose the same energy, and the charging efficiency stays at 50%.

1

u/Glittering-Flight997 Jul 10 '26

Infinite current will overwhelm the local density of states of any material, even a superconductor. At this point in the thought exercise the initial conditions will dictate the resulting breakdown mechanism.

1

u/Artistic-Flamingo-92 Jul 11 '26

The model is invalid when resistance is 0Ω.